PNP bipolar transistors are readily produced in CMOS integrated circuits as parasitic substrate devices. This is quite advantageous as bipolar transistors have an emitter-to-base voltage (Veb) that varies predictably with regard to temperature. The temperature of integrated circuits may thus be monitored using a bipolar transistor as a temperature transducer. Because a bipolar transistor is readily and inexpensively embedded with the circuits to be monitored, bipolar transistor temperature transducers are attractive options to an integrated circuit designer.
Although bipolar transistor temperature transducers are inexpensive, they suffer from a number of problems. For example, it is known that if the collector current for a bipolar transistor is changed from a first collector current value IC1 to a second collector current value IC2, a resulting change in emitter-to-base voltage (ΔVeb) is directly proportional to a product of the absolute temperature T and the logarithm of a ratio of the collector currents (IC2/IC1). If IC2 equals N*IC1, then the temperature is proportional to ΔVeb divided by the logarithm of N. This logarithm of N may be stored in a memory such that the temperature measurement merely requires mapping ΔVeb by some proportionality factor retrieved from the memory. The accuracy of the temperature measurement thus is a function of the accuracy for the collector current ratio. But the collector current IC for a PNP bipolar transistor temperature transducer is commonly determined from the emitter current IE. In particular, IC equals IE*(β/β+1), where β is a current gain factor that depends upon the collector current amplitude. A ratio of emitter currents will thus vary from the desired ratio of collector currents as a function of this variation in β. For high values of β, this dependence of the emitter current ratio on the variation in β leads to relatively little error. For example, a PNP bipolar transistor temperature transducer having a 10% variation for a β of 100 leads to just a 0.1° C. error as the collector current is increased tenfold. Such a relatively high value of β is readily achieved in discrete PNP transistors. But integrated PNP transistors typically have much lower values of β such as one or even less than one. In contrast to the high-β example, a PNP bipolar transistor temperature transducer having a 10% variation for a β of 1 leads to a 6° C. error as the collector current is increased tenfold.
To mitigate temperature measurement errors from β variation as a function of collector current, a lead may be used to monitor the collector current of the PNP device directly. While such a solution is feasible for discrete PNP devices, an integrated PNP device does not have a discrete collector terminal since its substrate serves as the collector terminal. Direct monitoring of the collector current for integrated PNP devices would thus be expensive and cumbersome because of the need for an additional pin on the transducer package and a corresponding trace on the package's printed circuit board.
Accordingly, there is a need in the art for an integrated bipolar transistor temperature transducer that compensates for the variation in β.
Embodiments of the present invention and their advantages are best understood by referring to the detailed description that follows. It should be appreciated that like reference numerals are used to identify like elements illustrated in one or more of the figures.